Resonant inductive coupling or magnetic phase synchronous coupling is a phenomenon with

inductive coupling In electrical engineering, two conductors are said to be inductively coupled or magnetically coupled when they are configured in a way such that change in current through one wire induces a voltage across the ends of the other wire through el ...

where the coupling becomes stronger when the "secondary" (load-bearing) side of the loosely coupled coil resonates. A

resonant transformer A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional part ...

of this type is often used in analog circuitry as a

bandpass filter A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. Description In electronics and signal processing, a filter is usually a two-po ...

. Resonant inductive coupling is also used in

wireless power Wireless power transfer (WPT), wireless power transmission, wireless energy transmission (WET), or electromagnetic power transfer is the transmission of electrical energy without wires as a physical link. In a wireless power transmission system, ...

systems for portable computers, phones, and vehicles.

Applications

Various resonant coupling systems in use or are under development for short range (up to 2 meters) wireless electricity systems to power laptops, tablets, smartphones,

robot vacuum A robotic vacuum cleaner, sometimes called a robovac or a roomba as a generic trademark, is an autonomous robotic vacuum cleaner which has a limited vacuum floor cleaning system combined with sensors and robotic drives with programmable controll ...

s, implanted medical devices, and vehicles like electric cars,

SCMaglev The SCMaglev (superconducting maglev, formerly called the MLU) is a magnetic levitation (maglev) railway system developed by Central Japan Railway Company (JR Central) and the Railway Technical Research Institute. On 21 April 2015, a manned se ...

trainsAbout the power supply on the car by induction current collecting method
/ref> and automated guided vehicles.Non-Contact Power Supply Transport System Technology of DAIFUKU AGV
since 1993 Specific technologies include: *

WiTricity WiTricity Corporation is an American wireless charging technology company based in Watertown, Massachusetts. The Massachusetts Institute of Technology (MIT) spin-off was founded by professor Marin Soljačić in 2007. WiTricity technology allows wi ...

* Rezence *

eCoupled eCoupled is a proprietary near-field wireless energy transfer technology developed by Fulton Innovation, a division of Alticor. It provides wireless power transfer via inductive coupling between a primary transmission coil and a secondary recei ...

Wireless Resonant Energy Link WREL (an acronym for Wireless Resonant Energy Link) is a form of wireless resonant energy transfer technology developed by Intel. The technology relies on strongly coupled based on resonant inductive coupling caused by electromagnetic resonato ...

(WREL) Other applications include: * Data transmission such as with passive RFID tags (for example in a

passport A passport is an official travel document issued by a government that contains a person's identity. A person with a passport can travel to and from foreign countries more easily and access consular assistance. A passport certifies the perso ...

) and

contactless smart card A contactless smart card is a contactless credential whose dimensions are credit-card size. Its embedded integrated circuits can store (and sometimes process) data and communicate with a terminal via NFC. Commonplace uses include transit ticke ...

s. * Resonant transformer of CCFL inverter that powers a cold-cathode fluorescent lamp. * Couple the stages of a

superheterodyne receiver A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carr ...

, where the selectivity of the receiver is provided by tuned transformers in the intermediate-frequency amplifiers. * High voltage (one million volt) sources for

X-ray X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...

production. The

Tesla coil A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low- current, high-frequency alternating-current electricity. Tesla experimented with a number of differe ...

is a resonant transformer circuit used to generate very high voltages, and is able to provide much higher current than high voltage

electrostatic machine An electrostatic generator, or electrostatic machine, is an electrical generator that produces '' static electricity'', or electricity at high voltage and low continuous current. The knowledge of static electricity dates back to the earliest ci ...

s such as the

Van de Graaff generator A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials. It produces very high voltage dire ...

.　However, this type of system radiates most of its energy into empty space, unlike modern wireless power systems which waste very little energy. Resonant transformers are widely used in

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transm ...

circuits as

s, and in switching power supplies.

History

In 1894

Nikola Tesla Nikola Tesla ( ; ,"Tesla"
'' In 1897 he patented a device called the high-voltage,

or "

." Transferring electrical energy from the primary coil to the secondary coil by resonant induction, a Tesla coil is capable of producing very high voltages at high frequency. The improved design allowed for the safe production and utilization of high-potential electrical currents, "without serious liability of the destruction of the apparatus itself and danger to persons approaching or handling it." In the early 1960s resonant inductive wireless energy transfer was used successfully in implantable medical devices including such devices as pacemakers and artificial hearts. While the early systems used a resonant receiver coil, later systems implemented resonant transmitter coils as well. These medical devices are designed for high efficiency using low power electronics while efficiently accommodating some misalignment and dynamic twisting of the coils. The separation between the coils in implantable applications is commonly less than 20 cm. Today resonant inductive energy transfer is regularly used for providing electric power in many commercially available medical implantable devices. Wireless electric energy transfer for experimentally powering electric automobiles and buses is a higher power application (>10 kW) of resonant inductive energy transfer. High power levels are required for rapid recharging and high energy transfer efficiency is required both for operational economy and to avoid negative environmental impact of the system. An experimental electrified roadway test track built circa 1990 achieved just above 60% energy efficiency while recharging the battery of a prototype bus at a specially equipped bus stop. The bus could be outfitted with a retractable receiving coil for greater coil clearance when moving. The gap between the transmit and receive coils was designed to be less than 10 cm when powered. In addition to buses the use of wireless transfer has been investigated for recharging electric automobiles in parking spots and garages as well. Some of these wireless resonant inductive devices operate at low milliwatt power levels and are battery powered. Others operate at higher kilowatt power levels. Current implantable medical and road electrification device designs achieve more than 75% transfer efficiency at an operating distance between the transmit and receive coils of less than 10 cm. In 1993, Professor John Boys and Professor Grant Covic, of the

University of Auckland , mottoeng = By natural ability and hard work , established = 1883; years ago , endowment = NZD $293 million (31 December 2021) , budget = NZD $1.281 billion (31 December 2021) , chancellor = Cecilia Tarrant , vice_chancellor = Dawn ...

in New Zealand, developed systems to transfer large amounts of energy across small air gaps.Wireless power Transfer: Introduction and History - Tutorial
CERV 2015 John Boys It was putting into practical use as the moving crane and the AGV non-contact power supply in Japan. In 1998, RFID tags were patented that were powered in this way. In November 2006, Marin Soljačić and other researchers at the

Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a Private university, private Land-grant university, land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern t ...

applied this near field behavior to wireless power transmission, based on strongly-coupled resonators. In a theoretical analysis, they demonstrate that, by designing electromagnetic resonators that suffer minimal loss due to radiation and absorption and have a near field with mid-range extent (namely a few times the resonator size), mid-range efficient wireless energy-transfer is possible. The reason is that, if two such

resonant circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ...

s tuned to the same frequency are within a fraction of a wavelength, their near fields (consisting of '

evanescent wave In electromagnetics, an evanescent field, or evanescent wave, is an oscillating electric and/or magnetic field that does not propagate as an electromagnetic wave but whose energy is spatially concentrated in the vicinity of the source (oscillati ...

s') couple by means of

evanescent wave coupling In electromagnetics, an evanescent field, or evanescent wave, is an oscillating electric and/or magnetic field that does not propagate as an electromagnetic wave but whose energy is spatially concentrated in the vicinity of the source (oscillati ...

. Oscillating waves develop between the inductors, which can allow the energy to transfer from one object to the other within times much shorter than all loss times, which were designed to be long, and thus with the maximum possible energy-transfer efficiency. Since the resonant wavelength is much larger than the resonators, the field can circumvent extraneous objects in the vicinity and thus this mid-range energy-transfer scheme does not require line-of-sight. By utilizing in particular the magnetic field to achieve the coupling, this method can be safe, since magnetic fields interact weakly with living organisms. Apple Inc. applied for a patent on the technology in 2010, after WiPower did so in 2008. In the past, the power source used on the JR Tokai SCMaglev car was generating with a gas turbine generator. In 2011, they succeeded in powering while driving (CWD:charge while driving) across a large gap by the JR Tokai proprietary 9.8 kHz phase synchronization technology developed based on technology similar to AGV's wireless power scheme. And the Japanese Ministry of Land, Infrastructure and Transportation evaluated the technology as all the problems for practical use were cleared. Construction of SCMaglev begin and commercial use will start in 2027.

Comparison with other technologies

Non-resonant

coupled inductors Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of the ...

, such as typical

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

s, work on the principle of a primary coil generating a

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and t ...

and a secondary coil subtending as much as possible of that field so that the power passing through the secondary is as close as possible to that of the primary. This requirement that the field be covered by the secondary results in very short range and usually requires a

magnetic core A magnetic core is a piece of magnetic material with a high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, ...

. Over greater distances the non-resonant induction method is highly inefficient and wastes the vast majority of the energy in resistive losses of the primary coil. Using resonance can help improve efficiency dramatically. If resonant coupling is used, the secondary coil is capacitive loaded so as to form a tuned LC circuit. If the primary coil is driven at the secondary side resonant frequency, it turns out that significant power may be transmitted between the coils over a range of a few times the coil diameters at reasonable efficiency. Compared to the costs associated with batteries, particularly non-rechargeable batteries, the costs of the batteries are hundreds of times higher. In situations where a source of power is available nearby, it can be a cheaper solution. In addition, whereas batteries need periodic maintenance and replacement, resonant energy transfer can be used instead. Batteries additionally generate pollution during their construction and their disposal which is largely avoided.

Regulations and safety

Unlike mains-wired equipment, no direct electrical connection is needed and hence equipment can be sealed to minimize the possibility of electric shock. Because the coupling is achieved using predominantly magnetic fields; the technology may be relatively safe. Safety standards and guidelines do exist in most countries for electromagnetic field exposures (e.g. ICNIRP ). Whether the system can meet the guidelines or the less stringent legal requirements depends on the delivered power and range from the transmitter. Maximum recommended B-field is a complicated function of frequency, the ICNIRP guidelines for example permit RMS fields of tens of microteslas below 100 kHz, falling with frequency to 200 nanoteslas in the VHF, and lower levels above 400 MHz, where body parts can sustain current loops comparable to a wavelength in diameter, and deep tissue energy absorption reaches a maximum. Deployed systems already generate magnetic fields, for example

induction cooker Induction cooking is performed using direct induction heating of cooking vessels, rather than relying on indirect radiation, Convection (heat transfer), convection, or thermal conduction. Induction cooking allows high power and very rapid increase ...

s in the tens of kHz where high fields are permitted, and

readers, where higher frequency is possible as the required energies are lower.

Mechanism details

Overview

This process occurs in a

, an electrical component which transformer consists of high Q coil wound on the same core with a

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of a ...

connected across a coil to make a coupled

LC circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ...

. The most basic resonant inductive coupling consists of one drive coil on the primary side and one resonance circuit on the secondary side.CERV 201
Wireless power Transfer: Introduction and History-Tutorial
, John Boys In this case, when the resonant state on the secondary side is observed from the primary side, two resonances as a pair are observed. One of them is called the antiresonant frequency (parallel resonant frequency 1), and the other is called the resonant frequency (serial resonant frequency 1'). The

short-circuit inductance Short-circuit inductance of a real linear two-winding transformer is inductance measured across the primary or secondary winding when the other winding is short-circuited.Japan Industrial StandarC 5602-1986, pp 34, 4305/ref>This value is sometimes ...

and resonant capacitor of the secondary coil are combined into a resonant circuit. When the primary coil is driven with a resonant frequency (serial resonant frequency) of the secondary side, the phases of the magnetic fields of the primary coil and the secondary coil are synchronized. As a result, the maximum voltage is generated on the secondary coil due to the increase of the mutual flux, and the copper loss of the primary coil is reduced, the heat generation is reduced, and the efficiency is relatively improved. The resonant inductive coupling is the near field wireless transmission of electrical energy between magnetically coupled coils, which is part of a

tuned to resonate at the same frequency as the driving frequency.

Coupling coefficient in the resonance state

In the transformer, only part of the flux generated by current through the primary coil is coupled to the secondary coil and vice versa. The part that couples is called ''mutual flux'' and the part that does not couple is called ''leakage flux''. When the system is not in the resonance state, this leads to the open-circuit voltage appearing at the secondary being less than predicted by the turns ratio of the coils. The degree of coupling is captured by a parameter called ''coupling coefficient''. The coupling coefficient, , is defined as the ratio of transformer open-circuit voltage ratio to the ratio that would be obtained if all the flux coupled from one coil to the other. However, if it is not open circuit, the flux ratio will change. The value of lies between 0 and ±1. Each coil inductance can be notionally divided into two parts in the proportions . These are respectively an inductance producing the mutual flux and an inductance producing the leakage flux. Coupling coefficient is a function of the geometry of the system. It is fixed by the positional relationship between the two coils. The coupling coefficient does not change between when the system is in the resonance state and when it is not in the resonance state, or even if the system is in resonance state and a secondary voltage larger than the turns ratio is generated. However, in the resonance case, the flux ratio changes and the mutual flux increases. Resonant systems are said to be tightly coupled, loosely coupled, critically coupled or overcoupled. Tight coupling is when the coupling coefficient is around 1 as with conventional iron-core transformers. Overcoupling is when the secondary coil is so close and the formation of mutual flux is hindered by the effect of antiresonance, and critical coupling is when the transfer in the passband is optimal. Loose coupling is when the coils are distant from each other, so that most of the flux misses the secondary. In Tesla coils around 0.2 is used, and at greater distances, for example for inductive wireless power transmission, it may be lower than 0.01.

Voltage gain (Type P-P)

Generally the voltage gain of non resonantly coupled coils is directly proportional to the square root of the ratio of secondary and primary inductances. :

A = k \sqrt \,

However, if in the state of resonant coupling, higher voltage is generated. The

L_sc2 on the secondary side can be obtained by the following formula. :

L_=(1-k^2)\cdot

The short-circuit inductance L_sc2 and the resonance capacitor Cr on the secondary side resonate. The resonance frequency ω₂ is as follows. :

\omega_2 =  =

Assuming that the load resistance is Rl, the Q value of the secondary resonance circuit is as follows. :

Q_2 = R_l \sqrt \,

The voltage generated in the resonance capacitor Cr at the peak of the resonance frequency is proportional to the Q value. Therefore, the voltage gain Ar of the secondary coil with respect to the primary coil when the system is resonating, :

A_r = kQ_2 \sqrt \,

In the case of the Type P-P, Q1 does not contribute to the voltage gain.

WiTricity type resonant inductive coupling system

The

type magnetic resonance is characterized in that the resonant coils on the primary side and the resonant coils on the secondary side are paired. The primary resonant coil increases the primary driving coil current and increases the generated magnetic flux around the primary resonator. This is equivalent to driving the primary coil at high voltage. In the case of the type on the left figure, the general principle is that if a given oscillating amount of energy (for example a pulse or a series of pulses) is placed into a primary coil which is capacitively loaded, the coil will 'ring', and form an oscillating magnetic field. Resonant transfer works by making a coil '' ring'' with an oscillating current. This generates an oscillating

. Because the coil is highly resonant, any energy placed in the coil dies away relatively slowly over very many cycles; but if a second coil is brought near it, the coil can pick up most of the energy before it is lost, even if it is some distance away. The fields used are predominantly non-radiative, near fields (sometimes called

s), as all hardware is kept well within the 1/4 wavelength distance they radiate little energy from the transmitter to infinity. The energy will transfer back and forth between the magnetic field in the inductor and the electric field across the capacitor at the resonant frequency. This oscillation will die away at a rate determined by the gain-bandwidth ( ''Q'' factor), mainly due to resistive and radiative losses. However, provided the secondary coil cuts enough of the field that it absorbs more energy than is lost in each cycle of the primary, then most of the energy can still be transferred. Because the ''Q'' factor can be very high, (experimentally around a thousand has been demonstratedWireless Power Transfer via Strongly Coupled Magnetic Resonances André Kurs, Aristeidis Karalis, Robert Moffatt, J. D. Joannopoulos, Peter Fisher, Marin Soljacic
/ref> with air cored coils) only a small percentage of the field has to be coupled from one coil to the other to achieve high efficiency, even though the field dies quickly with distance from a coil, the primary and secondary can be several diameters apart. It can be shown that a figure of merit for the efficiency is:WiTricity White Paper- Highly Resonant Wireless Power Transfer: Safe, Efficient, and Over Distance- Highly Resonant Wireless Power Transfer: Safe, Efficient, and Over Distance 2017 Morris Kesler
/ref> :

U = k \sqrt

Where ''Q₁'' and ''Q₂'' are the Q factors of the source and receiver coils respectively, and ''k'' is the coupling coefficient described above. And the maximum achievable efficiency is: :

\eta_ = \frac

Power transfer

Because the ''Q'' can be very high, even when low power is fed into the transmitter coil, a relatively intense field builds up over multiple cycles, which increases the power that can be received—at resonance far more power is in the oscillating field than is being fed into the coil, and the receiver coil receives a percentage of that.

Transmitter coils and circuitry

Unlike the multiple-layer secondary of a non-resonant transformer, coils for this purpose are often single layer

solenoids upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whose ...

(to minimise

skin effect Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the cond ...

and give improved ''Q'') in parallel with a suitable

. Alternative resonator geometries include wave-wound Litz wire and loop-gap resonators (LGRs). In the Litz wire-based resonators, insulation is either absent or low

permittivity In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ''ε'' ( epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more i ...

and low-loss materials such as

silk Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and is produced by certain insect larvae to form cocoons. The best-known silk is obtained from the ...

are used to minimise dielectric losses. The LGR geometries have the advantage that electric fields outside the resonator structure are very weak which minimizes human exposure to electric fields and makes the power transfer efficiency insensitive to nearby dielectrics. To progressively feed energy into the primary coil with each cycle, different circuits can be used. One circuit employs a Colpitts oscillator. In Tesla coils an intermittent switching system, a "circuit controller" or "break," is used to inject an impulsive signal into the primary coil; the secondary coil then rings and decays.

Receiver coils and circuitry

The secondary receiver coils are similar designs to the primary sending coils. Running the secondary at the same resonant frequency as the primary ensures that the secondary has a low impedance at the transmitter's frequency and that the energy is optimally absorbed. To remove energy from the secondary coil, different methods can be used, the AC can be used directly or rectified and a regulator circuit can be used to generate DC voltage.

References

External links

* – official site of a wireless power standard promoted by the Alliance for Wireless Power * – official site of a wireless power standard promoted by the Wireless Power Consortium * – official site of a wireless power standard promoted by the Power Matters Alliance
Instructables: wireless power
{{Nikola Tesla, state=collapsed Electric transformers Wireless energy transfer